As everybody knows, medicinal chemistry research has been dramatically transformed by biotechnology. Previously synthetic chemistry and natural products screening dominated drug research, but now molecular biology has become a driving force behind screening and the establishment of molecular targets.
Over the last years, most of the research in biotech companies has been directed to peptide and protein therapeutics in spite of problems associated with their low bioavailability, rapid metabolism, and lack of oral activity. Because of these limitations, research groups continue to rely upon chemical synthesis of nonpeptide substances for drug discovery, recognizing that small molecules are likely to remain the most viable avenue for the identification and optimization of potential drugs.
As is also known, combinatorial chemistry is a technique by which large chemical libraries can be generated by connecting together appropriate chemical building blocks in a systematic way. The ability to generate such large, chemically diverse libraries, either in a combinatiorial fashion or by any other high throughput parallel synthetic methods, combined with high throughput screening techniques, provides an immensely powerful tool for drug lead discovery and optimization.
Drug companies are increasingly interested in harnessing the ability of combinatorial synthesis to produce large collections (or libraries) of molecules to augment their existing sources of molecular diversity, and to fully exploit their capacity to capture millions of biological assay data points annually using high throughput robotic screening instrumentation.
This new science is still in its infancy, and to date most successful scaffold are derived from small heterocycles which are usually synthesized in very few steps. Thus several focused libraries have been built around bioactive cores such as benzodiazepines. However, this approach cannot be considered as a true method to generate innovative lead structures. Rather, it is a mean to optimize existing leads and is usually applied in drug development schemes.
Random libraries destined to search for innovative leads are very few today. As one example, a library based on diketopiperazine yielded a new submicromolar lead for a neurokinin-2 receptor after screening of this library on a variety of targets.
It is obvious that not all scaffolds may lead to potent drug candidates. Some very simple molecules requiring only one or two chemical steps may seem very attractive due to the huge size of the libraries that can be generated from them. Nonetheless, too simple molecules do not usually provide useful leads since they tend to lack target specificity, a prerequisite for a molecule to become a drug.
A class of organic structures with outstanding pharmaceutical activity has been termed as “macrocycle family”. Compounds like Taxol, Epothilone, Erythromycin, Neocarzinostatin, Rifampin and Amphotericin are either under clinical study or already marketed drugs and belong to this important family. Most of these products are of natural origin, since they are not usually tackled by medicinal chemists due to lack of knowledge associated with their synthesis.
Over the last years, the present inventors have developed expertise in the field of macrocycles synthesis. With such an expertise, they have developed a method of synthesis and evaluation of libraries of partially peptidic macrocycles which mimic β-turns, thereby making it possible to quickly explore huge quantities of conformationally restricted structures.